http-fetch: Fix object list corruption in fill_active_slots().

http-fetch: Fix object list corruption in fill_active_slots().

In fill_active_slots() -- if we find an object which has already arrived,
say as part of a pack, /don't/ remove it from the list.  It's already been
prefetched and someone will ask for it later.  Just label it as done and
carry blithely on.  (As it was, the code would dereference a freed object
to continue through the list anyway.)

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>
Signed-off-by: Junio C Hamano <junkio@cox.net>